Use of concomitant medications affecting calcium channels or those metabolized by the cytochrome p450 3A4 system including grapefruit juice, St. John's wort, HIV protease inhibitors, cimetidine, antibiotics (macrolides, fluoroquinones, antifungal, rifampin), antidepressants (fluvoxamine, norfluoxetine), barbiturates, anticonvulsants (carbamazepine, phenytoin), oral diabetes agents (pioglitazone, troglitazone), and glucocorticoids